In general, infrared radiation (IR) sensors are used in a variety of applications to detect infrared radiation and provide an electrical output that is a measure of the incident infrared radiation. IR sensors typically use either photonic detectors or thermal detectors for detecting the infrared radiation. Photon detectors detect incident photons by using the energy of the photons to excite charge carriers in a material. The excitation of the material is then detected electronically. Thermal detectors also detect photons. Thermal detectors, however, use the energy of said photons to increase the temperature of a component. By measuring the change in temperature, the intensity of the photons producing the change in temperature can be determined.
Photonic detectors typically have higher sensitivity and faster response times than thermal detectors. However, photon detectors must be cryogenically cooled in order to minimize thermal interference, thus increasing the cost, complexity, weight, and power consumption of the device. In contrast, thermal detectors operate at room temperature, thus avoiding the cooling required by photon detector devices. As a result, thermal detector devices can typically have smaller sizes, lower costs, and lower power consumption than photon detector devices.
One type of infrared thermal detector is a thermopile. A thermopile is formed of several thermocouples connected in series. Each thermocouple consists of two conductors of dissimilar materials that produce a voltage in the vicinity of a junction of the conductors that is dependent on the temperature difference between the junction and the other parts of the conductors. The thermocouples are connected in series with the “hot junctions” positioned nearest to the IR absorbing area of the detector and the “cold junctions” positioned farthest away from the IR absorbing area. To attain reasonable sensitivity in the thermopile-based IR detector, the hot and cold junctions of the thermopile need to be as thermally isolated as possible from each other and from other heat sources that can influence the temperatures of the hot and cold junctions. To achieve this thermal isolation, the thermopile is often placed on top of a dielectric layer on a substrate and a large back cavity is etched into the substrate beneath the thermopile to increase thermal resistance.
Another type of infrared thermal detector is a bolometer. A bolometer includes an absorber element for absorbing infrared radiation and a transducer element in thermal contact with the absorber element that has an electrical resistance that varies with temperature. In operation, infrared radiation incident upon the bolometer will be absorbed by the absorber element of the bolometer and the heat generated by the absorbed radiation will be transferred to the transducer element. As the transducer element heats in response to the absorbed radiation, the electrical resistance of the transducer element will change in a predetermined manner. By detecting changes in the electrical resistance, a measure of the incident infrared radiation can be obtained. Bolometers may serve as individual sensors, but may also be designed as rows or 2D arrays, referred to as microbolometer arrays.
Recent advances in technology have enabled the absorber element of a bolometer to be formed by atomic layer deposition (ALD). ALD enables absorber elements to be formed as thin metal films with precise and uniform thickness. As a result, ALD thin film bolometers are orders of magnitude more sensitive than thermopile sensors. The use of ALD thin film technology has allowed bolometer fabrication to be implemented on top of a complementary metal-oxide-semiconductor CMOS. However, there is still a need for methods of fabricating bolometer sensors that more fully integrate the design and structure of the bolometer into the CMOS process.